This has been in patch form for about a year, and I'm finally
getting around to applying it.  Basically, we separate the Cart
class into 3 classes:
  - Cart: stuff specific to cartridge emulation
  - BSTypes: various bankswitch types, both as enum and strings
  - CartDetector: actual bankswitch auto-detection code

Also made more use of smart pointers, and added const in a few
more places.

I promise to get back to the phosphor stuff soon.  This is just
to clear my head a little.